
If the begonia corms (that's what they're called rather than bulbs,) were planted in fresh soil, you can hold off fertilizing for 2 or 3 months, then go ahead and use dilute fertilizer -- 1/4 strength package directions. If the soil was not fresh, you can use dilute fertilizer now. Fertilize once a month to every week, depending on the amount of light (more light, more fertilizer.) Here's an article that will tell you more about growing begonias from corms: https://www.gardeningknowhow.com/ornamental/flowers/begonia/growing-begonias-from-corms.htm